The present invention relates generally to surface mount technologies, and, more particularly to the formation of electrically conductive bumps used in surface mount technologies (SMTs).
Flip-chip bumps are used to interconnect semiconductor devices, such as integrated circuit (IC) chips and microelectromechanical system (MEMS) devices, to an external substrate (e.g., a circuit board or another chip or wafer). Conventional bumps have been formed entirely of metal solder or another metal, such as copper, that is plated with solder, and are formed on the chip pads on the active side of the dies on the wafer during the final wafer processing steps.
When mounting the chip to external circuitry, the chip is turned or flipped over so that its active side faces down, and aligned so that its bumps align with matching pads on the external circuitry. The solder on the bumps is then flowed, through a process known as controlled collapse chip connection (C4), to form both a mechanical connection and an electrical connection with the external circuitry.
In implementations that utilize copper (or another metal), the copper acts as a pillar that provides a fixed stand-off between the chip and the external circuitry. To form the pillar, a layer of photoresist is spin coated over the entire wafer and cured, and, in a subsequent process, cylindrical channels are formed in the photoresist above the chip pads by photolithography and etching.
The channel acts as a mold that is filled with copper using electroplating to form the copper pillar. The channel also acts as a mold to electroplate a solder cap onto the top of the copper pillar. The remaining photoresist is then removed by etching to expose the copper pillar bump. During mounting, the solder cap is heated to its melting temperature, although not to the melting temperature of the copper pillar. It would be advantageous to have a more economical method of forming these electrically conductive bumps.